Electrical signaling system



' Oct. 9, 1934. L, E. RYALL. 3 M

ELECTRICAL S IGNALING SYSTEM Filed June 29, 1932 2 Sheets-Sheet lLennard Em R 511 Oct. 9, 1934. L. E. Wu. 199mm ELECTRICAL S IGNALINGSYSTEM Filed June 29, 1952 2 Sheets-Sheet 2 1mm Lennard Em? R5311Patented Oct. 9, 1934 PATENT OFFICE 1,976,096 ELECTRICAL SIGNALINGSYSTEM Leonard Ernest Ryall, Sutton, England, asslgnor to AssociatedElectric Laboratories, Inc., Ohicago, 111., a corporation of DelawareApplication June 29, 1932, Serial No.

In Great Britain July 15, 1931 16 Claims.

The present invention relates to electrical signaling systems and ismore particularly concerned with arrangements for discriminating betweensignals comprising currents of the same characteristics which differ,however, in the manner in which they cease. Thus, for example, wherealternating currents of voice frequency are employed for signaling overcircuits used also for the transmission of speech it is an object of theinvention to produce signal responding equipment which will operate onlywhen the currents stop abruptly, which is a condition impossible toattain with currents representing ordinary speech.

According to one feature of the invention in an electrical signalingsystem having signals transmitted in response to cessations of normalcurrent flow in the signaling circuit currents of similar character arearranged to operate receiving equipment differently according as theyare stopped suddenly or gradually.

According to another feature of the invention,

in a signaling system having signals transmitted in response tovariations in the fiow of direct current over a line the receivingequipment comprises two relays having different release times andsensitivities connected to the line in such manner that if the currentflow stops suddenly the relay which is quicker to release will releasefirst, while if the current falls gradually the relay having the shorterrelease time which is more sensitive is maintained operated until afterthe other relay has released.

According to a further feature of the invention in an electricalsignaling system employing voice frequency alternating currents forsignaling over a circuit which may be traversed by speech currents thereceiving apparatus for responding to signals includes a condenser whichis charged due 40 to the receipt of signaling currents and is connectedup in such manner that if the currents are interrupted suddenly theabsence of the signaling current in conjunction with the charged stateof the condenser operates the receiving apparatus while if the currentsare interrupted slowly aswould occur in the case of speech the condenserhas had time to discharge when the signal completely ceases and nooperation of the receiving apparatus results.

The invention will be better understood from the following descriptionof three methods of carrying it into effect which should be taken inconjunction with the accompanying drawings comprising Figs. 1, 2 and 3.

Referring first to Fig. 1, it is assumed that an alternating signalvoltage received across terminals 1, 2 is applied through a suitableimpedance 3 and a step-up transformer 4 to the grid of an amplifyingvalve 5, of which the normal gridcathode potential derived from battery6 is such 69 that a large current flows in the plate circuit, therebyholding relay 7 operated and the associated contacts 8, 9' open andclosed respectively. The amplified alternating current signal is appliedthrough condenser 10 across an impedance, which may if desired be aresonant circuit 11, 121, tuned to the required signal frequency. Thealternating voltage received across this impedance is rectified by anysuitable rectifier 13, for instance, of the metal oxide type, andapplied 7 across a condenser 14 which is shunted by a high resistance 15in parallel with a relatively low resistance 16. This voltage acrosscondenser 14 increases the negative grid-cathode voltage of valve 5,which causes the anode current to fall, 76 but the presence of theapplied signal prevents the anode current from falling sufficiently tocause relay '1 to release since the positive halves at least of thealternating signal voltage applied to the grid will be giving rise topulses of current 80 in the anode circuit. If the applied signal stopssuddenly, however, these pulses will cease but since the condenser 14 isnot at once discharged through resistance 16, the anode current fallssufficiently to release relay '7. Contact 8 in the local signal circuitcloses, whilst contact 9 opens so that condenser 14 can now dischargeonly through the high resistance 15, and thus the relay 7 remainsreleased for an appreciable period, for instance 2 seconds, until thecondenser 14 has discharged and thereby permitted the anode current torise again to a value sufficient to operate the relay. If on the otherhand the signal stops slowly, the condenser 14 discharges through thelow resistance 16 before the anode current has fallen sufiiciently topermit relay 7 to release and hence the contact 8 is not closed.

It may be pointed out that if a circuit tuned to the special signalfrequency is associated with the rectifier as in the arrangement shownin Fig. 1, the effect of currents of other frequencies which may bepresent in the received signal will be to increase the anode currentwithout supplying the extra negative grid potential so that thasecurrents will have no tendency to cause the release of relay '7.

It will be understood also that if there are a plurality of differentsignaling frequencies employed in respect of which signals may be sentsimultaneously, the transformer 4 will be pro-- vided with acorresponding number of secondary windings each connected to the grid ofthe valve in the corresponding receiving equipment- If the number ofdifferent frequencies is considerable however it may be preferable tomake use of simple filter circuits instead of the input transformer 4.so that each set of receiving equipment will be supplied with only arelatively small amount of signals of frequencies other than itsoperating frequency.

The arrangement illustrated in Fig. 3 shows a somewhat similar circuitwhich, however, does not make use of a rectifier for obtaining anegative grid cathode potential but instead obtains similar results byinitial amplification of the incoming signal so as to obtain a flow ofgrid current. The signal voltage applied to terminals 21 and 22 isextended to the series tuned circuit comprising condenser 23 andinductance 24 and is then amplified by the valve 25 the grid of which isconnected to the junction point of condenser 23 and inductance 24. Theanode circuit of the valve 25 includes the inductance 26 and isconnected to the grid of the valve 31 by way of the condenser 34,suitable grid bias being provided by the battery 32. As in the previousinstance the anode circuit of valve 31 includes a responding relay 27having contacts 28 and 29 and shunted by the condenser 30 to provide alow resistancepath for the alternating component of the current.Resistance 35 which is comparatively high corresponds to resistance 15in the previous arrangement whereas resistance 36 which is of much lowervalue is connected up by way of contacts 29 which are closed when relay2? is energized.

The operation of the circuit is similar to that previously described; anincoming signal is amplified by valve 25 to a value which causes gridcurrent to fiow and charge condenser 34. If the signal stops suddenly,the anode current falls sharply owing to the negative charge on its gridproduced by condenser 34 being no longer counteracted by the positivesignal half waves obtained from the anode circuit of valve 25 andaccordingly relay 27 releases. Thereupon the discharge circuit forcondenser 34 through the comparatively low resistance 36 is interruptedterywill be disconnected from both the condenser.

and discharge can then only take place comparatively slowly throughresistance 35. Accordingly relay 27 remains de-energized for anappreciable time to close a control circuit at contact 28. On the otherhand, if the signal stops slowly, con-= denser s4. is enabled todischarge through the comparatively low resistance 36 before theintermittent positive potential due to the signal is removed and hencerelay 27 remains operated.

The types of signal receiver Just described are suitable fordiscrimination between tone signals that can stop quickly, and speechsignals that may be of the same frequency as the tone signals, but whichdo not stop quickly. The application of the invention is not confined,however, to thisdiscrimination between tone and speech signals but itmay be used for discriminating between any made to stop slowly byincorporating in the transmitted signal circuit after the signal isstopped a suitable resonant circuit of low doc rement.

If a signal consisting of a train of impulses of the correct frequencyis transmitted, the receiver relay releases when the first impulseceases andcapacity condenser.

withsuitable impulse speed remains released until the next impulse isreceived. This impulse produces additional relay current,'which can bemade to cause the relay to operate again. The relay releases once morewhen the second impulse ceases. After the final impulse the receiverrelay remains released for a longer period and then attains its normaloperated state. It is found that impulses up to at least a frequency of20 per second may be successfully received in this manner.

Alternatively with a somewhat difl'erent adjustment oi the relay it maybe arranged that the relay remains released through the train ofimpulses and only re-operates when they cease, when there is suficienttime for the condenser to be substantially completely discharged.

The same principle of securing diiierent eflects according as a signalstops suddenly or slowly may beapplied to direct current circuits and asimple example of such an application is illustrated in Fig. 2. In thisfigure it is assumed that current from a suitable battery normally fiowsover the contacts of the key K to operate the relays A and B at thedistant end of the signaling line. Relay B is provided with a smallcopper slug to make it slightly slow to release and the adjustment ofthe two relays is such that relay B requires a larger current tomaintain it operated than relay A. The condenser CH which is preferablyan electrolytic condenser with a capacity of the order of 500 mi. isnormally connected across the battery and is thus fully charged. RelaysA and B are also normally operated whil the relayC is normallytie-energized.

If now the key K is operated to the right, the current flow over theline wilibe suddenly reduced to zero and relays A and B will release,

relay A before relay B in consequence of the lat ters copper slug. Inthese circtances no circuit is completed for relay C and the circuit 3to be controlled is closed-over contacts c2 and c3. When the key isagain restored to normal at the end of the signal period, relays A and Bare mth again operated and the conditions are restored to normal.

If now key K is operated to the left, the batand the line and thecondenser will'r. over the line, thereby securing that the time takenfor the current to fall to zero may be as much as one second. In these.circumstances in view of the fact that relay A will hold on a smallercurrent than relay B, relay B will be the first to de energize andconsequently a circuit will be com pleted for relay C which thereupon atarmature c1 completes a locking circuit for itself, at armature c2closes the circuit Y to be controlled and at armature 03 opens a pointin the circuit X so as to prevent this circuit being closed when relay Asubsequently falls away. As before, conditions are restored to normalwhen the key is released. It will be understood that the condenser CNneed not be permanently connected across the line as shown but may beconnected up only when a signal is to be sent, thereby permittingsatisfactory use of the line for other purposes, e. g.

In the case of direct current working the same principle could readilybe applied to'produce different efiects according as a current wasstarted slowly or suddenly the diflerences being obtained as in the casejust described by the use of a large What I claim as new and desire tosecure by Letters Patent is:

1. In an electrical signaling system, a signaling circuit, means fortransmitting signals in response to cessation of normal current flow insaid signaling circuit, and receiving equipment operated differently bycurrents of similar character in accordance with the sudden or gradualchange of the current flow.

2. In an electrical signaling system, a line, means for transmittingsignals in response to variations in the iiow of direct current oversaid line, a receiving equipment including two relays each havingdifferent release times and sensitivities, each of said relays connectedto said line,

the first of said relays having the quicker release time is releasedbefore the second relay in case the current flow stops suddenly and thesaid first relay is maintained operated until after the second relay isreleased in case the flow of current falls gradually.

3. In an electrical signaling system wherein voice frequency alternatingsignaling currents and speech currents are transmitted over a signalingcircuit, a receiving apparatus, a condenser in said receiving apparatuscharged in response to the receipt of said currents, means responsive tothe sudden interruption of said signaling currents and the charged stateof said condenser for operating said receiving apparatus, said receivingapparatus non-operative at the time the current flow ceases in case thesaid condenser is discharged by the gradual fall of current flow duringspeech transmission.

4. In an electrical signaling system wherein alternating signalingcurrents within the voice frequency range and currents of otherfrequencies are transmitted over a signaling circuit, a receivingapparatus including a condenser, a thermionic valve, a relay and ananode circuit, said thermionic valve including a suitable grid bias toeii'ect the normal operation of said relay in the anode circuit, acircuit tuned to the signaling frequency connecting the anode to thegrid circuit, circuit arrangements whereby the currents of any frequencyincreases the anode current while in consequence to the connections ofthe tuning circuit the currents of signaling frequency charges saidcondenser to increase the negative of the grid to reduce the anodecircuit, said relay released to operate said receiving apparatus only inresponse to the sudden cessation of said currents of signalingfrequency.

5. In an electrical signaling system wherein voice frequency alternatingsignaling currents are transmitted over asignaling circuit, a respondingequipment including a thermionic valve and a direct current relay, saidrelay connected in the anode circuit of said valve, a condenser includedin the grid circuit of said valve, means for charging said condenser inresponse to signaling current incoming over said signaling circuit, anda discharge circuit provided for said condenser over a circuit includinga low resistance and a normally closed contact on said relay.

6. In an electrical signaling system as claimed in claim 2 including athird relay, two circuits controlled by said third relay for signalingpurposes, and a circuit for controlling the operation of said thirdrelay controlled jointly by said two relays.

7. In an electrical signaling system as claimed in claim 2, meansincluding a charged condenser connected to the line for graduallyreducing the iiow of signal current.

8. In an electrical signaling system as claimed in claim 2, meansincluding a charged condenser connected to the line for graduallyreducing the flow of signal current, and means responsive to said lastmeans for transmitting a signal.

9. In an electrical signaling system as claimed in claim 3 including avalve for first amplifying the incoming signal and for charging the saidcondenser by the fiow of grid current due to the heavy input.

. 10. In an electrical signaling system as claimed in claim 4 includinga rectifier and means for charging said condenser by potential suppliedfrom the anode circuit by way of said rectifier.

11. In an electrical signaling system as claimed in claim 5 including arectifier, a circuit tuned to a particular signaling frequency, andmeans for operating said responding equipment only in response to saidparticular frequency by the association of said tuned circuit with saidrectifier.

12. In an electrical signaling system as claimed in claim 3 including ahigh resistance shunt permanently connected to said condenser.

13. In an electrical signaling system as claimed in claim 4 including ahigh resistance shunt permanently connected to said condenser formaintaining said relay released until said condenser has dischargedthrough said high resistance.

14. In an electrical signaling system as claimed in claim 4 includingcircuit arrangements whereby said relay is adjusted to reoperate inresponse to a further signal and also reoperate after a predeterminedinterval in case no signal is received

